Purification of propylene oxide resulting from epoxidation of propylene with hydrogen peroxide

ABSTRACT

A process of separating a purified propylene oxide from a crude epoxidation product produced, preferably, in an epoxidation reaction of propylene with hydrogen peroxide. The process involves removing bulk water, bulk methanol, and unreacted propylene from the crude epoxidation product and thereafter subjecting the resulting propylene oxide product to extractive distillation with water as an extraction solvent. Under distillation conditions, including a bottoms temperature of greater than about 55° C. and less than about 75° C., an overhead or side-cut distillate stream containing a purified propylene oxide is obtained with low yield loss of propylene oxide to propylene glycols and other glycol heavies. The purified propylene oxide can be further purified in a finishing distillation to obtain propylene oxide meeting commercial grade purity requirements.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 60/455506, filed Mar. 18, 2003.

BACKGROUND OF INVENTION

This invention pertains to a process of recovering propylene oxide inpurified form from an epoxidation reaction product, preferably, areaction product obtained from the epoxidation of propylene withhydrogen peroxide in the presence of a titanium-containing zeolitecatalyst.

Propylene oxide finds utility as a starting material in the preparationof polypropylene polyether polyols, which find utility in themanufacture of polyurethane polymers. Propylene oxide used for suchpurposes is required to meet strict purity specifications, so as toavoid disadvantageous effects in downstream polyurethane products.

Epoxidation processes comprising the reaction of propylene with hydrogenperoxide in the presence of titanium-containing zeolite catalysts areknown in the art, as illustrated, for example, in international patentpublication WO-A1-02/14298 (Enichem S. P. A.) Such processes aretypically conducted in the presence of a reaction solvent, preferably,methanol, which tends to promote high catalyst activity and selectivity.Such processes produce water as a co-product and minor amounts ofglycols, glycol ethers, acetaldehyde, acetone, and propionaldehyde asby-products. Accordingly, a crude propylene oxide product streamobtained from such epoxidation processes contains in addition to thepropylene oxide substantial quantities of reaction solvent, typicallymethanol, and water, as well as impurity amounts of glycols,acetaldehyde, acetone, and propionaldehyde.

The purification of the crude propylene oxide product is known to bedifficult. It is especially difficult to remove methanol andacetaldehyde to a high degree of efficiency. Nevertheless, commercialgrade propylene oxide requires that the product contains no greater than10 parts per million (ppm) methanol; no greater than 100 ppm water; andno greater than 30 ppm aldehydes.

The purification of propylene oxide has been considered in the priorart. Certain disclosures, represented by U.S. Pat. No. 4,140,588, teachthe extractive distillation of propylene oxide with water for thepurpose of removing contaminating quantities of methanol and acetone.Typically, a crude feed comprising from 92 to 99 percent propylene oxideand small quantities of water, methanol, and acetone is fed to a lowerportion of an extractive distillation zone. Water is fed at a higherpoint on the column, and the extractive distillation is operated at abottoms temperature ranging from 60° C. to 100° C. An overheaddistillate is obtained comprising a substantially pure propylene oxide;while a bottoms fraction is obtained comprising predominantly methanol,water, acetone, and disadvantageous amounts of propylene oxide andpropylene glycol. Typically, the disclosed process suffers a yield lossof propylene oxide that is greater than 1 percent and as high as 2.5percent.

Other art, represented by U.S. Pat. No. 5,849,938 and EP-B1-1,009,746,discloses the separation of propylene oxide, acetaldehyde, and methanolby extractive distillation using water or propylene glycol as extractionsolvent. The crude epoxidation mixture, containing from 2 to 10 percentpropylene oxide, 50 to 85 percent methanol, 10 to 30 percent water, and0.01 to 0.1 acetaldehyde by weight, is introduced into an intermediatesection of the distillation tower. A bottoms temperature from 90° C. to120° C. is maintained, such that a bottoms stream is obtained containingmethanol, water, any further extractive solvent, and a substantialportion of the acetaldehyde. A purified propylene oxide is obtained; butdisadvantageously, the concentration of methanol in the purifiedpropylene oxide remains higher than acceptable for most applications.Moreover, the references do not address the yield loss of propyleneoxide in the extractive distillation process due to side-reactions ofpropylene oxide with the extractive solvent and methanol in the feed.

Other art, represented by EP-A1-1,122,248 and WO-A1-01/57010, disclosesthe work-up of an epoxidation product stream containing propylene,propylene oxide, methanol, and an organic solvent, such as methanol. Thework-up comprises separating the product stream in a pre-evaporator suchthat between 20 and 60 percent of the amount of organic solvent and morethan 95 percent of the propylene oxide fed is removed with the overheadproduct. The residue of the organic solvent and over 90 percent of thewater fed is contained in the bottom product. Thereafter, the propyleneremaining in the overhead is stripped in a C3 stripper. The recoveredproduct mixture is thereafter subjected to extractive distillation usinga polar solvent, such as water, so as to obtain a purified propyleneoxide in the overhead product and methanol and polar solvent in thebottoms product. The references are silent with respect to variousaspects of the extractive distillation, the quality of purifiedpropylene oxide obtained, and the yield loss of propylene oxide.

Other art, represented by EP-B1-1003733 and U.S. Pat. No. 6,024,840,discloses the separation of methanol and acetaldehyde from a crudeepoxidation reaction product comprised of 2 to 10 percent propyleneoxide, 60 to 85 percent methanol, 10 to 25 percent water, 0.01 to 0.1percent acetaldehyde, and 0.01 to 0.1 propylene, by weight. It is taughtto fractionate the crude epoxidation reaction product at areflux:distillate ratio generally of from 10:1 to 30:1 to obtain abottoms stream comprising methanol, water, and at least 99 percent ofthe acetaldehyde, and to obtain an overhead stream comprising propyleneoxide, propylene, and residual methanol, but substantially devoid ofwater and acetaldehyde. It is further disclosed to remove propylene fromthe overhead stream in a second distillation. Thereafter, the resultingpropylene oxide stream devoid of propylene is taught to be subjected toextractive distillation using a polar solvent, such as propylene glycol,generally at a bottoms temperature of from 80° C. to 110° C. From theextractive distillation, a bottoms stream is obtained containing theextractive solvent, methanol, water, and other impurities; while apurified propylene oxide is obtained as overheads. Disadvantageously,the high bottoms temperature of the extractive distillation might causeunacceptable yield loss of propylene oxide due to by-product formation.Moreover, the concentration of methanol illustrated in the purifiedpropylene oxide is too high for most applications.

In consequence of the above, a need exists for improvements in theseparation and purification of propylene oxide reaction products,preferably those obtained from the epoxidation of propylene withhydrogen peroxide. An efficient and cost-effective separation schemethat produces commercial grade propylene oxide meeting required puritystandards would be highly desirable. Due to the high boiling point ofpropylene glycol, it would be more desirable if the separation methoddid not require this component as an extraction solvent. It would beeven more desirable if the separation method did not produceunacceptable losses of propylene oxide due to by-product formation withmethanol or extractive solvents in the crude product.

SUMMARY OF THE INVENTION

This invention provides for a novel process of separating a purifiedpropylene oxide from a propylene oxide reaction product, the processcomprising:

-   -   (a) introducing a reaction product comprising from about 65 to        88 percent propylene oxide, from about 10 to 35 percent        methanol, and less than 0.5 percent water, by weight, into a        bottom section of an extractive distillation zone;    -   (b) introducing water into an upper-half section of said        extractive distillation zone;    -   (c) removing from said extractive distillation zone under        distillation conditions a bottoms stream comprising propylene        oxide, water, and methanol,    -   (d) removing from said extractive distillation zone under        distillation conditions, an overhead or side-cut stream        comprising a purified propylene oxide essentially devoid of        methanol and water; the extractive distillation conditions being        sufficient to maintain a yield loss of propylene oxide of less        than about 0.3 mole percent.        For the purposes of this invention, the term “yield loss of        propylene oxide” will refer to the mole percentage of propylene        oxide lost in the extractive distillation process to glycols,        such as propylene glycol and glycol heavies (e.g., glycol        ethers), based on the total moles of propylene oxide fed to the        extractive distillation.

In a related aspect of this invention, the overhead or side-cut streamobtained from step (d) hereinabove, may optionally be subjected todistillation to recover a purified propylene oxide product meetingcommercial grade purity standards.

The novel process of this invention may be beneficially employed toseparate a purified propylene oxide from a reaction product obtainedfrom an epoxidation process, preferably, wherein propylene is epoxidizedwith hydrogen peroxide in the presence of a titanium-containing zeoliticcatalyst. Advantageously, the process of this invention achieves a highefficiency of separation yielding a purified propylene oxide that isessentially devoid of methanol and water. In a preferred embodiment ofthe invention, the purified propylene oxide may be distilled to removeany residual aldehydes that may be present, resulting in a purifiedpropylene oxide that meets commercial grade purity standards. Moreadvantageously, the process of this invention accomplishes thisefficient separation without undue loss of propylene oxide throughside-reactions to propylene glycol or propylene glycol heavies. Mostadvantageously, the novel process of this invention can be integratedinto a separation and purification scheme, described hereinafter, thatresults in a purified propylene oxide product of high quality beingrecovered from a crude epoxidation reaction product.

Accordingly, in a second aspect, this invention provides for anintegrated process of obtaining a purified propylene oxide from a crudeepoxidation reaction product, comprising:

-   -   (a) distilling a crude epoxidation reaction product comprising        propylene oxide, methanol, water, acetaldehyde, and unreacted        propylene under conditions sufficient to obtain a first bottoms        stream comprising a portion of the methanol, water, and        acetaldehyde, and a first overhead stream comprising propylene        oxide and unreacted propylene and the balance of the methanol,        water, and acetaldehyde;    -   (b) subjecting the first overhead stream from step (a) to        distillation under conditions sufficient to remove substantially        unreacted propylene and to obtain a second bottoms stream        comprising from about 65 to 88 percent propylene oxide, from        about 10 to about 35 percent methanol, less than about 0.5        percent water, from about 0.1 to about 0.5 percent acetaldehyde,        and less than about 2 percent unreacted propylene, by weight;    -   (c) feeding the second bottoms stream from step (b) to the        bottom section of an extractive distillation column and        subjecting the stream to extractive distillation with water as        an extractive solvent, under extractive distillation conditions        sufficient to obtain a third bottoms stream comprising propylene        oxide, water, and methanol; a third overhead or a side-cut        stream comprising a purified propylene oxide containing residual        acetaldehyde but essentially devoid of methanol, water, and        unreacted propylene; and optionally a top stream comprising        propylene; the yield loss of propylene oxide of step (c) being        maintained at less than about 0.3 mole percent;    -   (d) optionally, recycling the third bottoms stream from step (c)        to distillation step (a);    -   (e) optionally, recycling the top stream from step (c) to        distillation step (b); and    -   (f) optionally, distilling the third overhead or side-cut stream        from step (c) to recover a purified propylene oxide meeting        commercial grade purity standards.

In another aspect of this invention, the crude propylene oxide reactionproduct employed in the above-described separation and purificationprocesses is obtained from a process comprising contacting propylenewith hydrogen peroxide in a liquid phase in methanol solvent and in thepresence of an epoxidation catalyst under epoxidation conditionssufficient to prepare a crude epoxidation product comprising propyleneoxide, water, methanol, unreacted propylene, glycol and glycol ethers,and acetaldehyde. Unreacted hydrogen peroxide may also be present in thereaction product.

The aforementioned integrated process of this invention effects theseparation and purification of a crude epoxidation reaction productcomprising propylene oxide obtained in the epoxidation of propylene withhydrogen peroxide. Advantageously, the separation-purification processof this invention provides for a purified propylene oxide meetingcommercial grade purity standards. More advantageously, theseparation-purification process of this invention produces little, ifany, loss of propylene oxide product through side-reactions withmethanol or extractive solvent to form glycols and other heavies.

DRAWING

FIG. 1 illustrates a preferred embodiment of this invention wherein acrude propylene oxide reaction product, obtained from the epoxidation ofpropylene with hydrogen peroxide, is purified in aseparation-purification unit comprising four distillation towers, asdescribed in detail hereinafter.

DETAILED DESCRIPTION OF THE INVENTION

The novel process invention relates to a method of separating a purifiedpropylene oxide from a propylene oxide epoxidation product comprisingpropylene oxide, methanol, water, acetaldehyde, and unreacted propylene.The process beneficially achieves this desired goal without unacceptablelosses of propylene oxide to propylene glycol, glycol ethers and otherglycol heavies. These byproducts can arise during epoxidation to preparepropylene oxide as well as during the process of separating thepropylene oxide from the epoxidation reaction product. The process ofthis invention is concerned with maintaining low yield losses ofpropylene oxide to glycols during the separation of the propylene oxidefrom the epoxidation reaction mixture. In its broadest concept, thenovel process comprises:

-   -   (a) introducing a propylene oxide reaction product comprising        from about 65 to 88 percent propylene oxide, from about 10 to        about 35 percent methanol, and less than about 0.5 percent        water, by weight, into a bottom section of an extractive        distillation zone;    -   (b) introducing water into an upper-half section of said        extractive distillation zone;    -   (c) removing from said extractive distillation zone under        extractive distillation conditions a bottoms stream comprising        methanol, water, and propylene oxide;    -   (d) removing from said extractive distillation zone under        extractive distillation conditions an overhead or side-cut        stream comprising a purified propylene oxide essentially devoid        of methanol and water; the extractive distillation conditions        being sufficient to maintain a yield loss of propylene oxide of        less than about 0.3 mole percent.        The phrase “essentially devoid of methanol” shall be taken to        mean that the concentration of methanol in the overhead or        side-cut distillate stream comprises no greater than about 50        parts per million (ppm) methanol, preferably, no greater than        about 30 ppm, and more preferably, no greater than about 10 ppm        methanol, by weight. The phrase “essentially devoid of water”        shall be taken to mean that the concentration of water in the        overhead or side-cut distillate stream comprises no greater than        100 ppm water, by weight.

In a preferred embodiment, this invention provides for a novel processof separating a purified propylene oxide from a propylene oxideepoxidation product, comprising:

-   -   (a) introducing an epoxidation product comprising from about 65        to 88 percent propylene oxide, from about 10 to 35 percent        methanol, and less than about 0.5 percent water, by weight, to        an extractive distillation zone at about the first to fifth        theoretical stage measured from the bottom of the extractive        distillation zone;    -   (b) introducing water to the upper-half section of the        extractive distillation zone;    -   (c) maintaining the extraction distillation zone at a bottoms        temperature of about 55° C. to 75° C., so as to remove from said        extractive distillation zone under extractive distillation        conditions a bottoms stream comprising from about 20 to 40        percent propylene oxide, from about 10 to 25 percent water, and        from about 40 to 60 percent methanol, by weight, and;    -   (d) removing from said extractive distillation zone an overhead        or side-cut stream comprising greater than about 99 percent        propylene oxide, no greater than about 100 ppm water, and no        greater than about 50 ppm methanol, by weight; the extractive        distillation maintaining a yield loss of propylene oxide of less        than about 0.3 mole percent, preferably, less than about 0.2        mole percent, and more preferably, less than about 0.15 mole        percent.

In another related aspect of this invention, the overhead or side-cutstream comprising propylene oxide essentially devoid of methanol andwater may be further distilled to remove any residual aldehydes that maybe present, so as to recover a propylene oxide product meetingcommercial grade purity standards, such that the propylene oxide productcomprises greater than 99.95 percent propylene oxide and contains nogreater than 100 ppm water, no greater than 10 ppm methanol, and nogreater than 30 ppm aldehydes.

In a second aspect, this invention is an integrated process of preparinga purified propylene oxide comprising:

-   -   (a) distilling a crude propylene oxide reaction product        comprising propylene oxide, methanol, water, acetaldehyde, and        unreacted propylene to obtain a first bottoms stream comprising        a portion of the methanol, water, and acetaldehyde and a first        overhead stream comprising propylene oxide, unreacted propylene,        and the balance of methanol, water, and acetaldehyde;    -   (b) distilling the first overhead stream of step (a) to remove        substantially unreacted propylene as a second overhead stream        and to recover a second bottoms stream comprising from about 65        to 88 percent propylene oxide, from about 10 to 35 percent        methanol, less than about 0.5 percent water, from about 0.1 to        0.5 percent acetaldehyde, and less than about 2 percent        unreacted propylene, by weight;    -   (c) feeding the second bottoms stream obtained from step (b) to        the bottoms section of an extractive distillation column and        subjecting said stream to extractive distillation with water as        the extraction solvent under extractive distillation conditions        sufficient to obtain a third bottoms stream comprising propylene        oxide, water, and methanol; and a third overhead or side-cut        stream comprising a purified propylene oxide containing residual        acetaldehyde but essentially devoid of water, methanol, and        propylene; and optionally, a top stream comprising unreacted        propylene; wherein the yield loss of propylene oxide in the        extractive distillation step (c) is less than 0.3 mole percent;    -   (d) optionally, recycling the third bottoms stream from step (c)        to step (a);    -   (e) optionally, recycling the top stream from step (c) to step        (b); and    -   (f) optionally, distilling the third overhead or side-cut stream        from step (c) to obtain a purified propylene oxide of commercial        grade purity.        In step (a) the phrases “a portion of the methanol, water, and        acetaldehyde” and “the balance of the methanol, water, and        acetaldehyde” are described fully hereinbelow, wherein the        concentration ranges of methanol, water, and acetaldehyde in the        first bottoms and first overhead streams are set forth. The term        “residual acetaldehyde” shall be taken to mean a concentration        of acetaldehyde of less than 1 percent, preferably, less than        about 0.4 percent, by weight. In step (c), the phrase        “essentially devoid of methanol” shall be taken to mean that the        concentration of methanol in the overhead or side-cut distillate        stream comprises no greater than about 50 parts per million        (ppm) methanol, by weight. In step (c) the phrase “essentially        devoid of water” shall be taken to mean that the concentration        of water in the overhead or side-cut stream comprises no greater        than 100 ppm, by weight. A purified propylene oxide meeting        commercial grade purity standards shall consist essentially of        99.95 percent or greater of propylene oxide, but no greater than        100 ppm water, no greater than 10 ppm methanol, and no greater        than 30 ppm aldehydes, by weight.

In a more preferred embodiment, the extractive distillation step (c) isconducted at a bottoms temperature greater than about 55° C. and lessthan about 75° C. In another preferred embodiment, distillation step (a)to remove methanol and water is conducted at a reflux to distillateratio less than 10/1, more preferably, less than about 5/1, and mostpreferably, less than about 2/1.

The aforementioned crude propylene oxide reaction product is preferablyobtained by contacting propylene with hydrogen peroxide in a liquidphase in a solvent, preferably methanol, and in the presence of anepoxidation catalyst under epoxidation conditions sufficient to preparethe crude propylene oxide reaction product comprising propylene oxide,water, solvent (methanol), glycols, acetaldehyde, and unreactedpropylene. Other components may be present in the crude epoxidationproduct including, for example, lights, such as propane, andby-products, such as propionaldehyde and acetone. Generally, it isdesirable to manipulate the epoxidation process conditions to minimizethe production of by-products, such as glycols. Typically, theepoxidation catalyst is filtered from the crude propylene oxide reactionproduct prior to initiating the separation-purification scheme of thisinvention.

In reference to FIG. 1 herein, the novel process of this invention isillustrated in a preferred embodiment. A catalyst-free crude epoxidationreaction product, obtained from the epoxidation of propylene withhydrogen peroxide in methanol solvent and comprising propylene oxide,methanol, water, acetaldehyde, unreacted propylene, and optionallyglycols and glycol ethers formed during the epoxidation process, is fedvia line S1 into a first distillation tower T1 for the purpose ofremoving bulk methanol and water, and any glycols and glycol ethersformed during the epoxidation process. From the bottom of the firstdistillation tower there results a first bottoms stream S3 containingpredominantly methanol and water and minor amounts of glycol, glycolethers, acetaldehyde and other oxygenated byproducts. Only a portion ofthe acetaldehyde is typically obtained in the first bottoms stream withthe methanol and water. First bottoms stream S3 is typically sent to amethanol recovery unit (not shown in figure) for separation of methanolfrom water, glycols, and any other impurities. The recovered methanolcan be recycled as solvent to the epoxidation process. The recoveredwater can be disposed or recycled to the extractive distillation towerT3, as described hereinafter. From distillation tower T1, a firstoverhead stream, S2, containing propylene oxide, the balance of water,methanol, and acetaldehyde, unreacted propylene and typically anypropane present, is obtained and fed to distillation tower T2. A secondoverhead stream S4, obtained from distillation tower T2 and comprisingpropylene and optionally other lights, such as propane and oxygen, isrecycled, after propane and oxygen removal, as raw material back to theepoxidation reactor (not shown in figure). A second bottoms stream S5,obtained from second distillation tower T2 and comprising propyleneoxide, water, methanol, acetaldehyde, and less then 2% of propylene andpropane, is thereafter subjected to extractive distillation byintroducing second bottoms stream S5 into the bottom section of anextractive distillation tower T3. An extractive distillation solvent, inthis instance, water, is introduced into an upper-half section ofextractive distillation tower T3. Thereafter, under extractivedistillation conditions a third bottoms stream, S7, comprising propyleneoxide, water, methanol, and minor amount of glycols is removed from saidextractive distillation tower T3; stream S7 optionally may be recycledto first distillation tower T1 for recycling to theseparation-purification system. Additionally, in one preferredembodiment an overhead distillate stream S6, comprising propylene oxide,unreacted propylene and optionally other lights, such as propane, isobtained from extractive distillation tower T3. Stream S6 may berecycled to distillation tower T2 for recovery of propylene oxide,unreacted propylene and lights. Additionally, from extractivedistillation tower T3 a third overhead stream or side-cut stream, S8, isobtained comprising a purified propylene oxide optionally containingresidual acetaldehyde, the stream being essentially devoid of methanoland water. Third overhead stream or side-cut stream S8 is fed todistillation tower T4 to recover a fourth overhead stream S9 comprisingacetaldehyde, a fourth bottoms stream S11 (not shown) comprisingpropylene oxide and minor amounts of heavy impurities, which isoptionally recycled to tower T3, and a fifth bottoms or side-cut streamS10 comprising a highly purified propylene oxide meeting commercialgrade purity standards.

In accordance with the process of this invention, the crude epoxidationreaction product generally is obtained by epoxidizing propylene withhydrogen peroxide or an equivalent peroxide in the presence of anepoxidation catalyst in a reaction medium. Typically, the reactionmedium contains a solvent, preferably chosen as having a boiling pointbetween the boiling points of propylene oxide and water. Suitablesolvents include, inter alia, alcohols, for example, methanol, ethanol,or tert-butanol; ethers, for example, tetrahydrofuran or1,2-dimethylethane; and ketones, for example, acetone. In the process ofthis invention, methanol is preferably used as the solvent. Hydrogenperoxide is preferably employed as the oxidant and is usually providedas aqueous solution in a concentration from about 10 to 70 weightpercent, preferably, about 30 to 45 weight percent. Propylene may beused mixed with propane, typically in an amount from between about 0 and10 volume percent propane. Propylene is fed in excess relative tohydrogen peroxide. Typically on a molar basis, the propylene to hydrogenperoxide ratio is greater than about 2.5/1. The epoxidation catalyst,which typically comprises a titanium-containing zeolite, such astitanium silicalite, may be provided in a fixed-bed or alternativelysuspended in the reaction medium. The epoxidation reaction is typicallycarried out at temperatures between about 0° C. and 80° C. and atelevated pressures of about 10 bar (1,000 kPa) to 20 bar (2,000 kPa).Such processes are more fully described in the art, for example, asdisclosed in WO-A1-09/14298. The epoxidation catalyst is typicallyseparated from the crude epoxidation reaction product by filtration orother such means prior to implementation of the separation-purificationprocess of this invention. Typically, the catalyst-free crudeepoxidation reaction product comprises the following components, givenin percentages by weight:

-   -   Propylene oxide, about 3 to 35%    -   Methanol, about 35 to 80%    -   Water, about 8 to 40%    -   Acetaldehyde, about 0.01 to 0.1%    -   Propylene, about 0.5 to 15%    -   Propylene glycol and glycol heavies, about 0.1 to 1%

With reference again to FIG. 1, the catalyst-free crude epoxidationreaction product, in stream S1, is fed to distillation tower T1 andsubjected to fractionation under conditions sufficient to removesubstantially bulk methanol and water, glycols, a portion of theacetaldehyde, and other oxygenated impurities. Distillation tower T1typically comprises from greater than about 15 to less than about 40theoretical stages. The crude epoxidation reaction product is generallyintroduced into the upper ⅓ of the tower. The distillation operatesgenerally at a temperature in the top of the tower greater than about40° C. The distillation operates generally at a bottoms temperature ofless than about 100° C. The column top pressure is typically greaterthan about 1 bar abs (100 kPa). The column pressure is typically lessthan about 2.5 bar abs (250 kPa). Typically, tower T1 is operated at areflux to distillate ratio of less than 10/1, more preferably less thanabout 5/1, and most preferably, less than about 2/1.

A bottoms stream S3 comprising a fraction of the original methanol,water, and acetaldehyde, and essentially all of the glycols and glycolethers produced in the epoxidation process is obtained from tower T1.More specifically, greater than about 85 percent, by weight, of theoriginal methanol fed with the crude epoxidation product is removed inbottoms stream S3, while the remainder of the methanol remains withoverhead stream S2, also obtained from tower T1. More specifically,greater than about 90 percent and less than about 100 percent, byweight, of the original methanol fed to tower T1 is removed in bottomsstream S3, while the balance remains in propylene oxide overhead streamS2. Typically, greater than about 95 percent, and preferably, greaterthan about 99 percent, by weight, of the water fed with the crudeepoxidation product to tower T1 is removed with the bottoms stream S3.According to the invention, typically more than about 95 percent,preferably, more than about 98 percent, and more preferably, more thanabout 99 percent of the propylene oxide fed with the crude epoxidationproduct mixture to tower T1 is contained in the overhead stream S2. Theacetaldehyde is split between overhead stream S2 and bottoms stream S3.Typically, from about 30 to about 60 percent of the total acetaldehydepresent in the crude epoxidation reaction product remains in thepropylene oxide overhead stream S2; whereas the remainder of theacetaldehyde is removed with the bulk methanol and water in bottomsstream S3. Accordingly, overhead stream S2 comprises propylene oxide,methanol, a significantly reduced quantity of water, any unreactedpropylene, and roughly about one-half of the acetaldehyde fed.

Bottoms stream S3 comprising methanol, water, and a portion of theacetaldehyde and the overall amount of glycols fed, can be fractionatedin a methanol recovery unit (not shown in figure) to recover methanolfor recycle to the epoxidation process. Water obtained from the methanolrecovery unit may be disposed as waste or partially cycled todistillation tower T3 for use as an extractive distillation solvent.

Referring again to FIG. 1, following the removal of bulk methanol andwater in distillation tower T1, the overhead distillate stream S2 is fedto lights distillation tower T2 for the purpose of separating unreactedpropylene and other lights, including propane. The lights distillationtower T2 operates under conventional conditions for such a separation,including generally a temperature greater than about −25° C., andpreferably, greater than about −20° C. Typically, the lights tower T2operates at a temperature less than about 5° C., and preferably, lessthan about 0° C. The top pressure of distillation tower T2 is typicallygreater than about 2 bar abs (200 kPa) and less than about 5 bar abs(500kPa). From the second distillation tower T2, an overhead stream S4comprising propylene, propane, and other lights is recovered, which mayoptionally be recycled, typically after further purification, to theepoxidation reactor wherein propylene is consumed to form propyleneoxide. Following the separations of bulk methanol, bulk water, unreactedpropylene and other lights, and glycols and glycol heavies produced inthe epoxidation process, the propylene oxide epoxidation reactionproduct obtained from tower 12 as bottoms stream S5 typically comprisesa composition having the following components, in percentages by weight:

-   -   Propylene oxide, about 65 to 88%    -   Methanol, about 10 to 35%    -   Water, less than about 0.5%    -   Acetaldehyde, about 0.1 to 0.5%    -   Propylene, less than about 2%

Referring again to FIG. 1, bottoms stream S5 obtained from lightsdistillation tower T2 is fed to an extractive distillation tower T3,designed appropriately for a high efficiency separation of propyleneoxide from methanol and water. In a preferred embodiment, the extractivedistillation tower T3 contains greater than about 30 theoretical stages,and preferably, greater than about 50 theoretical stages. In a preferredembodiment, the extractive distillation tower T3 contains less thanabout 100 theoretical stages, and preferably, less than about 80theoretical stages. The use of one extractive distillation column ispreferred for economic reasons and ease of design; but the inventionshould not be limited to such a design. The propylene oxide reactionproduct, obtained as bottoms stream S5 and typically comprising fromabout 65 to 88 percent propylene oxide, from about 10 to about 35percent methanol, less than about 0.5 percent water, from about 0.1 toabout 0.5 percent acetaldehyde, and less than about 1 percent propylene,by weight, is beneficially introduced into the bottom section ofextractive distillation tower T3. For the purposes of this invention,the term “bottom section” shall be taken to mean the bottom ¼ of thecolumn, measured as theoretical stages from the bottom to the top of theextractive distillation zone. Preferably, the epoxidation reactionproduct is introduced into the extractive distillation column at a pointbetween about the first and fifth theoretical stage measured from thebottom. The extractive solvent, which in this case is water, isintroduced at a point in the upper-half of the column. A purifiedpropylene oxide distillate, shown as stream S8, is removed from the topor as a side-cut taken from about theoretical stages 2 to 6 of tower T3,measured from the top down towards the bottom of the column. If thepurified propylene oxide is taken as a side-cut, then the propylenereach in top product, shown as stream S6 in FIG. 1, optionally may berecycled to a propylene recovery system, for example, the second stagedistillation tower T2 described herein, for recycling of propylene tothe epoxidation process.

The extractive distillation operating parameters can be varied, providedthat the desired degree of purification of propylene oxide is effected.A suitable water to purified propylene oxide (PO) stream ratio(water/feed S8 ratio) is important in achieving optimum results in theextractive distillation. Generally, the water to purified PO streamratio will be greater than about 1:20, preferably, greater than about1:15 by weight. Generally, the water to purified PO stream ratio will beless than about 1:5, and preferably, less than about 1:8, by weight.Likewise, a suitable reflux to distillate ratio is important inachieving optimum results. For the case that purified propylene oxide istaken as top stream from the column T3, the reflux ratio generally willbe greater than about 3:1, preferably, greater than about 4:1; butgenerally less than about 10:1, and preferably, less than about 3:1, byweight.

The overhead temperature of extractive distillation column, shown astower T3 in FIG. 1, is typically maintained in a range that is greaterthan about 35° C. and less than about 45° C. The bottoms temperature ofthe extractive distillation tower is important in maintaining apropylene oxide yield loss typically of less than about 0.3 molepercent. Typically, the bottoms temperature of the extractivedistillation tower is maintained in a range that is greater than about55° C. and less than about 75° C. Typically, the extractive distillationtower is operated at a pressure greater than about 0.5 bar (50 kPa), andpreferably, greater than about 1 bar (100 kPa). Typically, theextractive distillation tower is operated at a pressure less than about2 bar (200 kPa), and preferably, less than about 1.6 bar (160 kPa).

By operating the extractive distillation tower under the aforementionedconditions, a bottoms stream is obtained that typically comprises thefollowing components, in percentages by weight:

-   -   Propylene Oxide, about 20-40%    -   Methanol, about 40-60%    -   Water, about 10-25%    -   Propylene glycol, about 0.05-0.3%    -   Other glycols and heavies, about 0.1-0.2%        Preferably, the bottoms stream, removed as stream S7 from the        extractive distillation tower T3, is combined with the crude        propylene oxide feed in stream S1 and recycled to first        distillation tower T1 for recycling through the        separation-purification process. Significantly, the total yield        loss of propylene oxide to propylene glycol and other glycol        heavies, obtained over the total process, is no greater than        about 0.3 mole percent, preferably, no greater than about 0.2        mole percent, and more preferably, no greater than about 0.15        mole percent. As mentioned hereinbefore, the term “yield loss of        propylene oxide” refers to the mole percentage of propylene        oxide lost in the extraction process to glycols and glycol        heavies, based on the total moles of propylene oxide fed to the        extractive distillation.

By operating the extractive distillation tower under the aforementionedprocess conditions, a purified propylene oxide stream is obtained as anoverhead or side-cut stream, which typically comprises the followingcomponents, in percentages by weight:

-   -   Propylene Oxide, greater than about 99.5%    -   Methanol, no greater than about 50 ppm    -   Water, no greater than about 100 ppm,    -   Acetaldehydes, no greater than 1%        Typically, however, the acetaldehyde in third overhead or        side-cut stream is greater than about 0.01 weight percent.        Preferably, the acetaldehyde is less than about 0.4 weight        percent.

Optionally, the purified propylene oxide obtained from the extractivedistillation tower T3 may be distilled in a conventional distillationcolumn to remove residual reaction by-products, in particularacetaldehyde, to yield a purified propylene oxide that meets commercialgrade standards of purity. This finishing distillation can be conductedin a tower having from about 40 to 80 theoretical stages and operatingat a temperature of between about 35° C. and 45° C., preferably about40° C., and a top column pressure of between about 1 bar abs (100 kPa)and about 3 bar abs (300 kPa), preferably, about 2 bar abs (200 kPa).The feed point can be, for example, at about the ⅓ point of the column,measuring from the top, plus or minus about 5 theoretical stages. Thehighly purified propylene oxide is obtained as bottoms stream S10. Forthe purposes of this invention, the “bottoms” cut may be taken from thebottom tray as well from the last 5 trays of the column. The purifiedpropylene oxide, taken as stream S10, consists essentially of thefollowing components, in percentages by weight:

-   -   Propylene oxide, greater than 99.95%    -   Water, no greater than 100 ppm    -   Methanol, no greater than 10 ppm    -   Acetaldehyde, no greater than 30 ppm

As seen from the above description, the process of this inventionadvantageously produces a purified propylene oxide meeting commercialgrade purity requirements without undue loss of propylene oxide topropylene glycol and other glycol heavies.

Throughout the description hereinabove, the words “no greater than” areused in reference to specified concentrations of particular productcomponents. For the purposes of this invention, the words “no greaterthan” shall be taken to mean a value equal to or less than the specifiedvalue. For example, the words “no greater than 100 ppm water” shall betaken to mean equal to or less than 100 ppm water.

The invention will be further clarified by a consideration of thefollowing example, which is intended to be purely exemplary of the useof the invention. Other embodiments of the invention will be apparent tothose of skill in the art from a consideration of this specification orpractice of the invention as disclosed herein.

EXAMPLE 1

With reference to FIG. 1, a crude reaction product mixture resultingfrom the epoxidation of propylene with hydrogen peroxide in methanolsolvent in the presence of titanium silicalite catalyst was filtered toremove the catalyst and thereafter fed as stream S1 to distillationtower T1. The crude epoxidation product had the composition shown inTable 1, in percentages by weight: TABLE 1 Crude Epoxidation ProductComponent Wt % Propylene Oxide 5.5 Methanol 78.1 Water 9.3 Propylene 5.3Propane 0.8 Propylene glycol & 0.6 glycol ethers Acetaldehyde 0.04 Otherimpurities 0.36

The operating conditions for tower T1 were as follows: top temperature,40° C.; top pressure, 2 bar (200 kPa); bottom temperature, 89° C. Fromtower T1 was obtained a first bottoms fraction, shown as stream S3,containing methanol, water, acetaldehyde, and heavy components, such asglycol and glycol ethers; and a first light fraction as stream S2containing propylene oxide, unreacted propylene, and the balance ofmethanol, water, and acetaldehyde. The light fraction was furtherfractionated in a second column (T2) containing 20 theoretical stagesand operating at −12.5° C. on the top and 4 bar (400 kPa) to removeunreacted propylene as second light stream S4. A second bottoms fractionwas obtained as bottoms steam S5 having the following composition, inpercentages by weight, as shown in Table 2: TABLE 2 Bottoms Stream S5Bottoms Component (wt %) Propylene Oxide 86.69 Methanol 10.25 Water 0.12Propylene 0.62 Acetaldehyde 0.26

The propylene oxide reaction product having the composition shown inTable 2, obtained as second bottoms stream S5, was fed to an extractivedistillation column (T3) containing 60 theoretical stages. The feedpoint of the reaction product was at theoretical stage 2 from the bottomof the column. Water was used as an extractive solvent, being added attheoretical stage 30, measured from the bottom to the top of the column.The bottom temperature of the column was maintained at 61° C. The toptemperature was maintained at 37° C. The weight ratio of water asextractive solvent to purified propylene oxide was 1:12.5. The topcolumn pressure was maintained at 1.3 bar abs (130 kPa). A purifiedpropylene oxide stream was removed as stream S8 as a side cut fromtheoretical stage 3 of extractive distillation tower T3, measured fromthe top of the column to the bottom. A bottoms stream was obtained asstream S7. The compositions of the overhead stream S8 and bottoms streamS7 were found to be the following, as shown in Tables 3 and 4,respectively: TABLE 3 Overheads Stream S8 Component Overheads (wt %)Propylene Oxide 99.700 Methanol 0.001 Water 0.006 Propylene 0.003Acetaldehyde 0.285

TABLE 4 Bottoms Stream S7 Component Bottoms (wt %) Propylene Oxide 32.52Methanol 45.20 Water 20.65 Propylene glycol 0.25 Glycol ethers 0.15Acetaldehyde 0.075From Table 3 it is seen that the overhead stream S8 obtained from theextractive distillation column contained propylene oxide essentiallydevoid of methanol and water (methanol only 10 ppm; water only 60 ppm).From Table 4 it is seen that the bottoms stream S7 containedpredominantly propylene oxide, methanol, and water. The quantity ofpropylene oxide lost to propylene glycol corresponded to 0.125 kg/h,equivalent to 0.25 weight percent. The quantity of propylene oxide lostto glycol ethers and other glycol heavies corresponded to 0.075 kg/h,equivalent to 0.15 weight percent. At a production rate of 125 kg/hpropylene oxide, the total loss to propylene glycol and glycol etherswas evaluated to be 0.115 mole percent, based on the total moles ofpropylene oxide and glycols produced from the extractive distillation.

The bottoms stream S7 obtained from extractive distillation tower T3 wasrecycled to stream S1 feeding to tower T1. The purified propylene oxidestream, taken as overheads stream S8 from the extractive distillationcolumn T3, can be fed to a finishing distillation tower T4 to remove lowboiling components, particularly acetaldehyde. The finishingdistillation was conducted in a tower having 60 theoretical plates andoperating at a temperature of 40° C. and a top column pressure of 2 bar(200 kPa). The feed point was theoretical stage 15 measured from the topof the column to the bottom. A finished propylene oxide, which meetscommercial grade purity standards, was obtained as bottoms productstream S10 having the composition as shown in Table 5. TABLE 5 FinishedPropylene Oxide Component Overhead¹ Propylene Oxide 99.97 wt % Methanol 10 ppm Water 100 ppm Acetaldehyde  30 ppm^(1.)Maximum observed for methanol, water, acetaldehyde,

1. A process of separating a purified propylene oxide from a propyleneoxide reaction product, the process comprising: (a) introducing areaction product comprising from about 65 to 88 percent propylene oxide,from about 10 to 35 percent methanol, and less than about 0.5 percentwater into a bottom section of an extractive distillation zone; (b)introducing water into an intermediate section of said extractivedistillation zone; (c) removing from said extractive distillation zoneunder distillation conditions a bottoms stream comprising propyleneoxide, water, and methanol, (d) removing from said extractivedistillation zone under distillation conditions, an overhead or side-cutstream, comprising a purified propylene oxide essentially devoid ofmethanol and water; the extractive distillation conditions beingsufficient to maintain a yield loss of propylene oxide of less thanabout 0.3 mole percent.
 2. The process of claim 1 wherein the propyleneoxide reaction product fed to the extractive distillation zoneadditionally comprises less than 2 percent propylene and/or from about0.1 to 0.5 percent acetaldehyde, by weight.
 3. The process of claim 1wherein the extractive distillation zone contains from greater thanabout 30 to less than about 100 theoretical stages.
 4. The process ofclaim 1 wherein the propylene oxide reaction product is fed to thebottom ¼ of the extractive distillation zone, measured as theoreticalstages from the bottom to the top of the extractive distillation zone.5. The process of claim 1 wherein water is introduced into theupper-half section of the extractive distillation zone.
 6. The processof claim I wherein the extractive distillation zone is operated at awater to purified propylene oxide (PO) stream ratio of greater thanabout 1:20 and less than about 1:5, by weight.
 7. The process of claim 1wherein the distillation zone is operated at an overhead temperature ofgreater than about 35° C. and less than about 45° C.
 8. The process ofclaim 1 wherein the distillation zone is operated at a bottomstemperature of greater than about 55° C. and less than about 75° C. 9.The process of claim 1 wherein the distillation zone is operated at apressure of greater than about 0.5 bar (50 kPa) and less than about 2bar (200 kPa).
 10. The process of claim 1 wherein a bottoms stream isobtained that comprises the following components, in percentages byweight: from about 20 to 40 percent propylene oxide, from about 40 to 60percent methanol, from about 10 to 25 percent water, from about 0.05 to0.3 percent propylene glycol, and from about 0.1 to 0.2 percent otherglycols and heavies.
 11. The process of claim 1 wherein the purifiedpropylene oxide overhead or side-cut stream comprises the followingcomponents, in percentages by weight: greater than about 99.5 percentpropylene oxide, no greater than about 50 ppm methanol, and no greaterthan about 100 ppm water.
 12. The process of claim 1 wherein the yieldloss of propylene oxide to propylene glycol and other glycol heavies isless than about 0.2 mole percent.
 13. The process of claim 1 wherein thepropylene oxide obtained as an overhead or side-cut stream from theextractive distillation zone is distilled to yield a purified propyleneoxide meeting commercial grade purity standards.
 14. The process ofclaim 13 wherein the purified propylene oxide meeting commercial gradestandards of purity is comprised of the following components, inpercentages by weight: propylene oxide, greater than 99.95 percent; nogreater than 100 ppm water; no greater than 10 ppm methanol; and nogreater than 30 ppm aldehydes.
 15. The process of claim 1 wherein thepropylene oxide reaction product is obtained from a process comprisingcontacting propylene with hydrogen peroxide in a liquid phase inmethanol solvent and in the presence of an epoxidation catalyst underepoxidation conditions.
 16. A process of separating a purified propyleneoxide product obtained from the reaction of propylene with hydrogenperoxide in the presence of a titanium-containing catalyst, theseparation-purification process comprising: (a) introducing anepoxidation product comprising from about 65 to 88 percent propyleneoxide, from about 10 to 35 percent methanol, and less than about 0.5percent water, by weight, to about the first to fifth theoretical stagemeasured from the bottom of an extractive distillation zone; (b)introducing water into the upper-half of the extractive distillationzone; (c) maintaining the extractive distillation zone at a bottomstemperature greater than about 55° C. and less than about 75° C., so asto remove from said extractive distillation zone under extractivedistillation conditions a bottoms stream comprising from about 20 to 40percent propylene oxide, from about 10 to 25 percent water, and fromabout 40 to 60 percent methanol, by weight, and; (d) removing from saidextractive distillation zone an overhead or side-cut stream comprisinggreater than 99.5 percent propylene oxide, no greater than about 100 ppmwater, and no greater than about 50 ppm methanol, by weight; whilemaintaining a yield loss of propylene oxide of less than about 0.3 molepercent.
 17. A process of separating a purified propylene oxide from anepoxidation reaction product, the process comprising: (a) distilling ina first distillation zone a crude propylene oxide reaction productcomprising propylene oxide, methanol, water, acetaldehyde, and unreactedpropylene to obtain a first bottoms stream comprising a portion of themethanol, water, and acetaldehyde, and a first overhead streamcomprising propylene oxide and unreacted propylene and the balance ofthe methanol, water, and acetaldehyde; (b) distilling in a seconddistillation zone the first overhead stream of step (a) to removeunreacted propylene and to recover a second bottoms stream comprisingfrom about 65 to 88 percent propylene oxide, from about 10 to 35 percentmethanol, less than about 0.5 percent water, and from about 0.1 to 0.5percent acetaldehyde, and less than 2 percent unreacted propylene, byweight; (c) feeding the second bottoms stream obtained from step (b) tothe bottoms section of an extractive distillation column and subjectingsaid stream to extractive distillation with water as the extractionsolvent under extractive distillation conditions sufficient to obtain athird bottoms stream comprising propylene oxide, water, and methanol;optionally, a top stream comprising unreacted propylene; and a thirdoverhead distillate or side-cut stream comprising a purified propyleneoxide containing residual acetaldehyde but essentially devoid of water,methanol, and unreacted propylene, while maintaining a yield loss ofpropylene oxide in the extractive distillation step (c) of less than 0.3mole percent; (d) optionally, recycling the third bottoms stream fromstep (d) to step (a); (e) optionally, recycling the top stream from step(c) to step (b); and (f) optionally, distilling the third overhead orside-cut distillate stream from step (c) to remove any residualacetaldehyde and to obtain a purified propylene oxide of commercialgrade purity.
 18. The process of claim 17 wherein the crude propyleneoxide is obtained in an epoxidation process comprising reactingpropylene with hydrogen peroxide in the presence of atitanium-containing catalyst.
 19. The process of claim 17 wherein thecrude propylene oxide epoxidation product comprises the followingcomposition in percentages by weight: from about 3 to 35 percentpropylene oxide, from about 35 to 80 percent methanol, from about 8 to40 percent water, from about 0.5 to 15 percent propylene, and less thanabout 0.1 percent acetaldehyde.
 20. The process of claim 18 wherein theepoxidation catalyst is a titanium silicate.
 21. An apparatus forseparating and purifying a crude propylene oxide reaction product, theapparatus comprising: (a) a first distillation tower into which a crudeepoxidation product comprising propylene oxide, methanol, water,acetaldehyde, unreacted propylene is fed; said first tower functioningto remove a portion of the methanol, water, and acetaldehyde in a firstbottoms stream and to produce a first overhead stream comprisingpropylene oxide and unreacted propylene and the balance of methanol,water, and acetaldehyde; (b) a second distillation tower into which thefirst overhead stream is fed; said second tower functioning to removesubstantially unreacted propylene and to produce a second bottoms streamcomprising from about 65 to 88 percent propylene oxide, from about 10 to35 percent methanol, less than about 0.5 percent water, from about 0.1to about 0.5 percent acetaldehyde, and less than about 2 percentunreacted propylene; (c) a third distillation tower into which thesecond bottoms stream is fed and into which an extractive solvent isfed; said third tower finctioning as an extractive distillation zone toremove essentially all of the remaining methanol and water, so as toproduce a third bottoms stream comprising methanol and water; a thirdoverhead or a side-cut stream comprising a purified propylene oxidecontaining residual acetaldehyde but essentially devoid of water andmethanol; and optionally, a top stream comprising residual unreactedpropylene; (d) optionally, a means for recycling the third bottomsstream to the first distillation tower; (e) optionally, a means forrecycling the third overhead stream to the second distillation tower;and (f) optionally, a fourth distillation tower into which the thirdoverhead or side-cut stream is fed; said fourth tower functioning toremove any residual acetaldehyde and to produce a purified propyleneoxide meeting commercial grade purity standards.